334 AGRICULTURAL BACTERIOLOGY. 



but plainly dependent upon proper conditions of life. Let the 

 conditions be normal and the animal may resist the attack of 

 parasitic bacteria ; but let them become abnormal, so as to re- 

 duce its vitality, and the animal is much more likely to suc- 

 cumb. 



For example : it is a well-known fact that tuberculosis is 

 much more prevalent .among cattle that are kept stabled most 

 of the time, than among those who live a considerable portion 

 of the time in the open air. Now this may be due, in part, to 

 the fact that stabled cattle have a greater chance of acquiring 

 the contagion, since the animals are kept so close together. But 

 this is certainly not the whole reason. Young cattle that are 

 kept in the open for a year or two are less liable to take the 

 disease than those kept in the stable, even though subsequently 

 put under similar conditions. In localities where the animals 

 run out of doors all the time the disease is rare. The more 

 closely the animals are housed the greater the tendency to 

 this disease, and it is practically certain that this greater ten- 

 dency of stall-fed animals to yield to the disease is not because 

 they are so much more likely to be infected, but because of 

 the depressing influence which such a sedentary life has upon 

 the vitality of the animal, reducing its resisting powers. It is 

 also a general belief that highly bred cattle have a greater 

 tendency to this disease than less highly bred stock. Stated 

 in this way the conception may not be correct ; but it is practi- 

 cally certain that animals that have been bred for the purpose 

 of producing great quantities of milk are rather more liable to 

 yield to disease than those not so highly specialized. Such a 

 specialization of the vitality in the direction of an abnormally 

 high action of the milk glands cannot fail to be at the expense 

 of other vital functions. These breeds have been developed 

 in one direction until they have become abnormal. It is not 

 to be wondered at if such an abnormal development should 

 have resulted in the reduction of their general vitality, and of 



